The efficient catalytic trimerization of olefinic monomers, such as the trimerization of ethylene to 1-hexene, is an area of great interest for the production of olefinic trimers of varying degrees of commercial value. In particular, 1-hexene is a valuable comonomer for linear low-density polyethylene (LLDPE). 1-hexene can also be produced by a conventional transition metal oligomerization process, although the trimerization route is preferred as it largely avoids the production of unwanted olefins.
Several different catalytic systems have been disclosed in the art for the trimerization of ethylene to 1-hexene. A number of these catalysts are based on chromium.
U.S. Pat. No. 5,198,563 (Phillips) discloses chromium-based catalysts containing monodentate amine ligands useful for trimerizing olefins.
U.S. Pat. No. 5,968,866 (Phillips) discloses an ethylene oligomerization/trimerization process which uses a catalyst comprising a chromium complex which contains a coordinating asymmetric tridentate phosphane, arsane or stibane ligand and an aluminoxane to produce alpha-olefins which are enriched in 1-hexene.
U.S. Pat. No. 5,523,507 (Phillips) discloses a catalyst based on a chromium source, a 2,5-dimethylpyrrole ligand and an alkyl aluminium activator for use in the trimerization of ethylene to 1-hexene.
Chem. Commun., 2002, 8, 858-859 (BP), discloses chromium complexes of ligands of the type Ar2PN(Me)PAr2 (Ar=ortho-methoxy-substituted aryl group) as catalysts for the trimerization of ethylene.
U.S. 2003/0166456, which is hereby incorporated by reference in its entirety, (BP) discloses a catalyst for the trimerization of olefins comprising a source of chromium, molybdenum or tungsten, a ligand containing at least one phosphorus, arsenic or antimony atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, and optionally an activator. The ligand used in most of the examples is (2-methoxyphenyl)2PN(Me)P(2-methoxyphenyl)2.
Although the catalysts disclosed in the BP documents mentioned above have good selectivity for 1-hexene within the C6 fraction, a relatively high level of by-product formation (e.g. decenes) is observed. It would therefore be desirable to provide a catalyst for the trimerization of olefinic monomers, especially for the trimerization of ethylene to 1-hexene, which reduces by-product formation (e.g. decenes) while maintaining selectivity for 1-hexene.
It has now been surprisingly found that the catalyst compositions and processes of the present invention provide an efficient route for the selective production of 1-hexene from ethylene while reducing the level of by-product formation, especially C10.